Thinking about a new bike through cycle to work. Would be for commuting, on a mix of road and cycle paths, and bimbling with the family on cycle paths or at the most towpaths or forest roads. It would need to be capable of having a child seat on the back, and I notice that the roadrat has rack mounts. Any alternatives I could consider? Kona do a nice SS cyclocross bike, but not sure if that takes a rack, doesn't look like it from the pics. Or a Langster, not as expensive?

Just got one of these and very happy with it: http://www.genesisbikes.co.uk/bikes/cross/day-one/day-one/overview/

On One Pompino?

Rack mounts and wotnot. Cheap. Versatile. Road/off road. Job's a good un.

That genesis one looks nice. I see there's one with drop bars which I find more comfortable.

What's with that £100 odd quid to buy an on-one with a cyclescheme voucher?

I'm pleased with my Kona Dew Plus; it gets used mostly for road commuting, but also carries my daughter on a baby seat and has been down the occasional fire-road and bridleway.

They even do a drop handle-bar version;
http://www.konaworld.com/bike.cfm?content=dewdrop

I have a drop bar alfine Roadrat purchased from sideways cycles on the cyclescheme and whilst it's early days so far….I love it. Jack of all trades, master of none and all that, ugly and unrefined, pretty much perfect for me! Muduards, a big fat saddle bag and non skinny tyres and I'm pretty much sorted. My kids will certainly be passengers on this in due course.

And the £100 quid, that's On One passing back the bit that cyclescheme charge/steal from them!

Assuming you haven't already got a seat you don't need rack mounts for a child seat. Humax seats bolt on to your downtube only. We've got one and it's very good.
I'd still get a rat though, great bikes.

Any road bike with sufficient clearance for tyres up to 26 or – if poss – 28.

From what you've described, I'd ride my winter road bike (Giant SCR with full guards and rack mount) with its 24c tyres. It gets ridden off road loads.